Explanation: Until the mid-20th century the largest impediment to trade between area "1" and area "2" on the map was climate. The Sahara Desert was a major impediment to trade. True, there were Trans-Saharan trade routes for centuries, but there is no denying that the desert was an inconvenience at the very least.
The case of the extraneous variable is most likely to be determined by the student grade level that is level of the education program that is for the grade level. This refers to all the course level and can be use to trace out the external variable that are not intentionally studied in the test or experiment and tends to affect the other variables.
